1. Field of the Invention
The present invention relates to data storage systems, and more particularly to data storage systems that store data in tracks on a patterned media. Still more particularly, the present invention relates to a method and system for radial and circumferential alignment of data tracks on patterned media.
2. Description of the Prior Art
Data storage systems record and reproduce data stored on a recording medium. Conventional systems typically include one or more storage disks in the storage system. The recording medium generally takes the form of a circular storage disk having a plurality of concentric data tracks formed thereon. Magnetic and optical disks are two examples of storage disks that are used in data storage systems.
The positioning of one or more recording heads relative to the data tracks is generally accomplished by incorporating a closed-loop, electro-mechanical servo system. The implementation of the servo system may include a dedicated servo surface that is associated with one of the plurality of heads in the data storage system. Alternatively, short bursts of servo data, referred to as a servo burst field, may be written amid the contents of the user data tracks. One technique used to form the servo data on a disk is to use a servo track writer. A servo track writer writes the servo data and a control system monitors the servo surface or the servo burst field data to maintain the position of the heads relative to the concentric tracks on the disk(s). Unfortunately, as track density increases, conventional servo track writers are unable to write servo data with the necessary accuracy.
One method for improving the servo data density on a storage disk is to use patterned media. A patterned media is a storage disk having a combination of raised features and depressed features in or on the surface of the disk. The servo marks are formed on each surface of a disk before the disk is built into a storage system. When fabricating patterned disks, typically one surface of a disk is patterned first, and when the patterning process for the first surface is complete, the pattern is then formed on the second surface of the disk.
The disks are built into a storage system after all of the disk surfaces have been patterned. The disks are typically placed on a shaft that fits into a hole at the center of each disk. The disks are then clamped into place in order to secure each disk in a fixed plane, one above the other. Each disk will then rotate around the shaft at its corresponding center.
Unfortunately, the positioning of the disks relative to the shaft is influenced by various tolerances, including shaft and clamp tolerances. FIG. 1 depicts a data track written on a patterned storage disk. A patterned storage disk 100 is rotated by a spindle motor mounted at the center 102 of the storage disk 100. The solid line 104 represents the ideal track. The dashed line 106 represents the actual center of the track written on the disk after the disk is installed in the storage system. As can been seen, the track 106 is not a concentric circle on the disk. Instead, track 106 has been written eccentric in relation to the center of the disk 100 (or to the center of balance of the disk).
Additionally, the tracks are not necessarily lined up from one disk surface to another disk surface. This is known as radial misalignment. Radial misalignment and eccentricity mean the servo system has to work harder to follow the track and to find the same track on another disk.
Another limitation to patterned storage disks is that once the disks are assembled into the storage system, the sectors may not be aligned from one disk to another. FIG. 2 illustrates a sector on two disks in a conventional data storage system. Storage disk 200 is positioned directly above storage disk 202 in the data storage system. Storage disk 200 has a sector 206 located at a particular location on the surface of the storage disk 200. Ideally, sector 206 should be located at the same particular point (point 208) on storage disk 202. Unfortunately, due to circumferential misalignment, sector 206 is actually located at another location on the disk 202. This means that when the recording head is switched in order to read (or write) data on disk 202, the sector number on the surface of disk 202 can be independent of the last sector number obtained from the surface of disk 200. Circumferential misalignment can significantly increase access times and reduce the performance of the system.
The present invention overcomes the limitations of the prior art by providing a method and system for radial and circumferential alignment of data tracks. The radial alignment is accomplished by first calibrating the system to determine radial correction factors. The servo system is then activated, and the final servo patterns are written using the radial correction factors. The final servo patterns are written as concentric tracks on each disk in the data storage system.
The calibration for the radial alignment begins by keeping the head in a fixed position. The position information is read from the disk and a radial correction factor is calculated for each ruler on the disk. A ruler is a position-sensing pattern that defines the radial position of a recording head. The rulers are patterned onto each disk before the disks are assembled into a storage system. The radial correction factors are then used by the servo system when writing the final servo patterns to write concentric tracks on the disk. The radial correction factors are added to the measured position information to form a feedback signal that is input into a controller to cancel the eccentricity and other disturbances.
The number of times a head travels over a boundary is taken into account when accurately determining the position of the head during the calibration process. The variable k denotes the ruler number and Qk denotes the quadrant at the kth ruler. The variable T denotes the number of wrap-overs from quadrant 4 to 1 minus the number of wrap-overs from quadrant 1 to 4. Initially, the system waits until the ruler number equals zero, and then sets T equal to zero. If Qk equals one and Qkxe2x88x921 equals four, then T=T+1. If however, If Qk equals four and Qkxe2x88x921, equals one, then T=Txe2x88x921. The correction factor wk is then computed at block by adding T to the measured position information mk. If the ruler number (k) is less than the number of rulers (n), the variable k is incremented and the system waits for the next ruler. The process ends when a correction factor wk has been determined for each ruler.
In another aspect of the invention, the circumferential offsets of the disks in the data storage system are determined. In the exemplary embodiment described herein, the number of servo sectors are numbered from zero (0) to m, and the number of heads and their corresponding disk surfaces are numbered from zero (0) to h. The variable km represents the sector number provided by the pattern, and the variable kc denotes the corrected sector number.
The process begins by switching to head 0 and waiting until sector 0 is detected. Once sector 0 is detected, the system switches to head i (where 0xe2x89xa6ixe2x89xa6h). The circumferential correction factor ci is then set to the number of the next sector detected on head i. This procedure repeats for each head in the system (except head 0). A corrected sector number is then determined by redefining the original sector numbers using the circumferential correction factors. During normal operation of the storage system, the corrected sector numbers are determined by the equation kc=mod (kmxe2x88x92ci+m, m), where mod (a,b) denotes the remainder of the division a/b. The corrected sector numbers are used to provide circumferential position information for the read and write operations.